I was at a house party last night, and my only connection was to the host through a friend who left early. After my friend left, I was more or less a random.

Thankfully everybody there was pretty easygoing, and lots of friend groups didn't know each other. I met new people fairly easily. However, lots of people did know each other, and as I was talking to girls I felt weird escalating beyond conversation and basic touching because I felt like an interloper. I didn't get much response, and where I might otherwise push I just let it slide. Nobody was making out or anything, so I wasn't sure what sort of moves would be frowned upon in this crowd if a stranger made them.

Anybody have insights to share from experiences like this?

I would say most people are pretty friendly. If you're a friendly guy, and get to know people (ie. figure out what they like to do, what they are interested in) and then converse with them about those subjects, they'll reciprocate. Then vwal-la! You have friends at the party...possibly even new friends. Feel free to make plans with these people, trade numbers, if you find you have something in common with them if you want. Get to know girls the same way, and if there's sexual interest there, get her number and meet her again if you don't feel comfortable escalating. If you and the girl really seem like you're hitting it off, suggest that you and her go back to your place, or go get some drinks or something, or some food or dessert, and then vwal-la, you're on a date with a girl late at night who's most likely drunk and in a party-mood...need I say more?

As for hooking up at the party...read the environment I would say. If people are hooking up - go for it, if not, see previous paragraph. I would say you made the right call in this situation not to hook up, but still pussied out a little and could have made friends/gotten numbers/whatever.